Method of machining silicon wafer

ABSTRACT

A method of machining a silicon wafer is capable of preventing the generation of cracks, which start from a tapered or bevel portion in the periphery of the thin silicon wafer, when the wafer of a single crystal body is machined into the thin wafer. A plurality of silicon wafers  1  having a bevel portion  2  in the periphery are laminated, the bevel portions  2  in the peripheries of the plurality of laminated silicon wafers are ground at the same time, and a face of each silicon wafer is ground so as to obtain a thin silicon wafer  10.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of machining asingle-crystal silicon wafer used for semiconductor elements orsemiconductor substrates. More particularly, the present inventionrelates to a method of machining a silicon wafer having a bevel portionin the periphery thereof so that the silicon wafer can be formed into athin silicon wafer.

[0003] 2. Description of the Related Art

[0004] A silicon wafer used for a semiconductor element or semiconductorsubstrate is cut out from a columnar single crystal body to adisk-shape. To stabilize the profile and to make it easy to handle thesilicon wafer, the silicon wafer has a bevel portion or chamferedportion (referred to as a bevel portion hereinafter) in its periphery.Thickness of the silicon wafer, which has been cut out from a singlecrystal body to a disk shape, is usually about 0.7 mm. However, when thesilicon wafer is actually used, the thickness of the silicon wafer isreduced, by means of back-grinding, to several tens of μm.

[0005] As shown in FIGS. 1 and 2, the above thin silicon wafer 1 isconventionally obtained as follows. The silicon wafer having the bevelportion 2 in its periphery is sucked and held by the sucking stage 3.Under the above condition, back-grinding is conducted on the siliconwafer by a grinding device not shown in the drawing so as to reduce thethickness of the silicon wafer to a range from about 0.7 mm to severaltens of μm. In this way, the thin silicon wafer 10 is conventionallyprovided.

[0006] In the process of back-grinding, when the silicon wafer is suckedand held by the sucking stage, in some cases, a protective tape 4 isbonded to the side of the silicon wafer coming into contact with thesucking stage.

[0007] Irrespective of the existence of the protective tape 4, thefollowing problems may be encountered. In the bevel portion in theperiphery of the silicon wafer 1, 10, what is called a “gap portion” 5,in which the silicon wafer does not come into contact with the suckingface of the sucking stage 3, is generated. Therefore, a distribution ofthe sucking force or sucking pressure, which is generated between thethin silicon wafer 10 and the sucking table 3, cannot be uniform allover the surface of the disk-shaped silicon wafer 10. As a result, afterthe completion of back-grinding, thickness of the silicon wafer 10cannot be accurately uniform all over the surface. In the case ofhandling the thin silicon wafer 10 which has been back-ground, andespecially in the case of using the wafer for the three-dimensionalmounting in which plating is required and in the case of using the waferfor a silicon interposer, as the thickness of the periphery of thesilicon wafer is small and the bevel portion of the wafer is tapered,the wafer tends to crack and handling of the wafer is difficult. In anextreme case, the entire wafer of a crystal body is broken becausecracks start from this tapered portion.

[0008] The typical prior art is disclosed in the official gazette ofJapanese Unexamined Patent Publication No. 62-154614. In this officialgazette, there is disclosed a technique in which a laminated body, whichis made when mirror faces of a plurality of semiconductor wafers arecontacted with each other, is ground so as to simultaneously grind theplurality of semiconductor wafers.

[0009] The official gazette of Japanese Unexamined Patent PublicationNo. 63-307200 discloses the following technique. A surface of the waferis roughly ground, and then the surface of the wafer is subjected tolapping. Next, a plurality of such the wafers are put on each other, andonly the side surfaces of the wafers are etched. Then, the entire facesof the individual wafers are etched and polished.

[0010] In the official gazette of Japanese Unexamined Patent PublicationNo. 5-243208, there is disclosed a technique in which an etchingsolution is supplied into a processing tank and the corners at theperiphery of the silicon wafer are etched.

[0011] In the official gazette of Japanese Unexamined Patent PublicationNo. 10-242439, there is disclosed a technique in which the bonded wafersare oxidized at the temperature of 1000 to 1200° C. in the wetatmosphere so as to grow the silicon oxide film and fill a gap portionformed between the bonded wafers and, then, an outer circumferentialedge portion of the wafer is ground.

[0012] In the official gazette of Japanese Unexamined Patent PublicationNo. 2001-113447, the following technique is disclosed. In an edgegrinding device for grinding an edge of the semiconductor wafer, aminute gap is formed between a rotary column of the grinding mechanismand a laminated body of the semiconductor wafers. A pressured grindingsolution is introduced into the minute gap, and grinding is highlyaccurately and uniformly conducted on an outer circumferential edgeportion of the semiconductor wafer.

[0013] According to the prior art shown in FIGS. 1 and 2, the followingproblems may be encountered. In the bevel portion in the periphery ofthe silicon wafer, what is called “a gap portion” is formed which doesnot come into contact with the sucking face of the sucking stage.Therefore, the distribution of the sucking force or pressure generatedbetween the silicon wafer and the sucking table does not become uniformall over the face of the silicon wafer. Accordingly, the thickness ofthe thin wafer, which has been back-ground, cannot be uniform. Further,as the bevel portion in the periphery of the silicon wafer is tapered,the wafer tends to crack, and handling of the silicon wafer becomesdifficult.

SUMMARY OF THE INVENTION

[0014] Therefore, it is an object of the present invention to provide amethod of machining a thin silicon wafer capable of preventing thegeneration of cracks, which start from a tapered portion in theperiphery of the thin silicon wafer, when the wafer of a single crystalbody is machined into the thin wafer.

[0015] In order to accomplish the above object, according to the presentinvention, there is provided a method of machining a thin silicon wafercomprising the following steps of: laminating a plurality of siliconwafers each providing with a body having front and back surfaces and aperiphery having a bevel portion; grinding the peripheries of theplurality of laminated silicon wafers to remove the bevel portions fromthe bodies of the silicon wafers; and dividing the laminated siliconwafers into individual silicon wafers and grinding the back surface ofeach of the silicon wafers to obtain a thin silicon wafer. When thegrinding machine side or the sucking table side is rotated, the siliconwafer can be back-ground.

[0016] The step of grinding the back surface of each of the siliconwafers can be conducted while the front surface of the silicon wafer isbeing sucked and held by a sucking table. When the grinding machine sideor the sucking table side is rotated, the silicon wafer can beback-ground.

[0017] The step of grinding the back surface of each of the siliconwafers can be conducted while a protective film is bonded to the frontsurface of the silicon wafer which comes into contact with the suckingtable. When the above protective film is bonded, it is possible toprotect the front surface of the silicon wafer which is not subjected tomachining.

[0018] The sucking table is made of a porous material and a sucking faceof the sucking table has an outer diameter slightly larger than that ofthe body of the silicon wafer, after the bevel portion is removed. Dueto the foregoing, the entire surface of the silicon wafer can beuniformly sucked and held by the sucking table.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a sectional view showing a state of a silicon waferbefore it is back-ground by the conventional thin silicon wafermachining method;

[0020]FIG. 2 is a sectional view showing a state of a silicon waferafter it is back-ground by the conventional thin silicon wafer machiningmethod;

[0021] FIGS. 3(a) and 3(b) are respectively a perspective view and asectional view of a silicon wafer before it is machined;

[0022]FIG. 4 is a perspective view showing a laminated silicon wafers;

[0023]FIG. 5 is a perspective view showing a state in which peripheralbevel portions of a plurality of silicon wafers, which are a laminatedbody, are ground;

[0024]FIG. 6 is a perspective view showing a state before and after eachsilicon wafer is back-ground;

[0025]FIG. 7 is a partial sectional view showing a state before asilicon wafer of the present invention is background; and

[0026]FIG. 8 is a partial sectional view showing a state in whichthickness of a silicon wafer of the present invention is reduced afterback-grinding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Referring to the accompanying drawings, an embodiment of thepresent invention will be explained in detail.

[0028] FIGS. 3 to 6 are views showing a form of a wafer in each step ofmachining the thin silicon wafer of the present invention.

[0029]FIG. 3(a) is a perspective view of the silicon wafer 1, which iscut out to a disk-shape from a columnar single crystal body used for asemiconductor wafer, wherein this silicon wafer has not been machinedyet. At the periphery of the disk-shaped silicon wafer 1, there isprovided a bevel portion 2 so that the profile can be stabilized and thehandling property can be enhanced. Thickness of this silicon wafer isapproximately 0.7 mm. As shown in FIG. 3(b) which shows a cross-sectionof the bevel portion 2, the peripheral portion is tapered. As shown inFIG. 1, the peripheral portion may be chamfered when it is viewed in thecross sectional view.

[0030]FIG. 4 is a view showing a state of the laminated body 20 in whicha plurality of silicon wafers 1, which have been cut out into a diskshape and have not yet been back-ground, are laminated on each other insuch a manner that the mirror faces of the silicon wafers 1 come intocontact with each other. The number of the silicon wafers to belaminated is usually 10 to 50.

[0031]FIG. 5 is a view showing a state in which the bevel portions inthe peripheries of a plurality of silicon wafers 1, which are thelaminated body 20, are ground at the same time so that the plurality ofsilicon wafers 1 are formed into a cylindrical laminated body 30. Anouter diameter of each silicon wafer 1 is reduced so that the bevelportion in the periphery of the silicon wafer 1 can be completelyremoved. In the case of grinding, it is possible to use an appropriatecutting device (not shown) having a cutting face parallel with thecentral axis of the laminated body. Concerning this type grindingdevice, for example, it is possible to use the device described in theofficial gazette of Japanese Unexamined Patent Publication No.2001-113447.

[0032]FIG. 6 is a view showing an individual silicon wafer which isseparated from the laminated body 30, the peripheral bevel portion ofwhich has been cut off, wherein FIG. 6 shows the states before and afterback-grounding. Before back-grinding is conducted, the thickness of thesilicon wafer 1 is approximately 0.7 mm as described above. However,when the silicon wafer 1 is subjected to back-grinding, the thickness ofthe wafer is reduced to about several tens of μm.

[0033] Next, referring to FIGS. 7 and 8, the step of back-grinding willbe explained in detail below. FIG. 7 is a sectional view of the siliconwafer of the present invention before back-grinding is conducted. FIG. 8is a sectional view of the silicon wafer of the present invention, afterback-grinding is conducted, in which thickness of the silicon wafer isreduced.

[0034] When the silicon wafer 1 is sucked and held on the sucking stage3 of the back-grinding device, the protective tape 4, the size of whichis substantially the same as that of the silicon wafer, the peripheralbevel portion of which has been cut, is bonded onto the side 40 of thesilicon wafer opposite to the cutting side. Then, the protective tape 4side of the silicon wafer 1 is set on the sucking stage 3 of theback-grinding device.

[0035] The sucking table 3 or the sucking stage is made of porousmaterial. Although not shown in the drawing, a pump to suck air andpiping are provided. Therefore, in the case of a sucking action, theentire surface of the sucking stage 3 is uniformly given negativepressure so that a sucking force necessary for holding the silicon wafer1 on the sucking face of the sucking table can be generated.

[0036] The sucking table 3 has a sucking face, the outer diameter ofwhich is a little larger than the outer diameter of the silicon wafer 1,the peripheral bevel portion of which has been ground and removed. Dueto the foregoing, it is possible for the sucking table 3 to stably holdthe entire face, of the silicon wafer 1, by a sucking force uniformlygiven to the entire surface.

[0037] Even when the above protective tape 4 is not arranged, itpossible for the sucking table 3 to hold the silicon wafer 1 on thesucking table 3 and no problems are actually caused in the process ofback-grinding. Irrespective of the existence of the protective tape 4,there are no “gap portions” which might be caused by “peripheralprotrusions” such as a bevel portion in the periphery of the siliconwafer 1, 10. Therefore, a sucking force or pressure generated betweenthe silicon wafer 1, 10 and the sucking table becomes uniform all overthe surface of the silicon wafer 1, 10. Accordingly, thickness of thesilicon wafer can be made uniform, over the entire surface, in theprocess of back-grinding.

[0038] In the case where the thin silicon wafer 10 is handled after thecompletion of back-grinding, especially in the case of athree-dimensional mounting wafer and a silicon interposer which need tobe plated with metal, as no tapered portions, such as a bevel portion,are arranged in the periphery of the silicon wafer, the thin siliconwafer 10 can be very easily handled. Therefore, it is possible to solvethe problems of cracks caused in the silicon wafer which is acrystalline body.

[0039] The embodiment of the present invention is explained above byreferring to the accompanying drawings. However, it should be noted thatthe present invention is not limited to the above specific embodiment,and variations, modifications and corrections may be made by one skilledin the art without departing from the spirit and scope of the presentinvention.

[0040] As described above, according to the present invention, there isprovided a method of machining a thin silicon wafer comprising the stepsof: laminating a plurality of silicon wafers having a bevel portion inthe periphery; grinding the bevel portions in the peripheries of theplurality of laminated silicon wafers at the same time; andback-grinding a face of each silicon wafer so as to obtain a thinsilicon wafer. Therefore, no bevel portions exist in the periphery ofthe silicon wafer when back-grinding is conducted. Therefore, thesilicon wafer is closely contacted with the sucking face of the suckingstage. Accordingly, no floating portions occur between the silicon waferand the sucking table, and the distribution of sucking pressuregenerated between the silicon wafer and the sucking table becomesuniform over the entire surface of the silicon wafer. Therefore, it ispossible to provide an effect that the thickness of the thin siliconwafer, which has been subjected to back-grinding, can be made uniformand the thin silicon wafer can be easily handled.

1. A method of machining a thin silicon wafer comprising the followingsteps of: laminating a plurality of silicon wafers each providing with abody having front and back surfaces and a periphery having a bevelportion; grinding the peripheries of the plurality of laminated siliconwafers to remove the bevel portions from the bodies of the siliconwafers; and dividing the laminated silicon wafers into individualsilicon wafers and grinding the back surface of each of the siliconwafers to obtain a thin silicon wafer.
 2. A method as set forth in claim1, wherein the step of grinding the back surface of each of the siliconwafers is conducted while the front surface of the silicon wafer isbeing sucked and held by a sucking table.
 3. A method as set forth inclaim 1, wherein the step of grinding the back surface of each of thesilicon wafers is conducted while a protective film is bonded to thefront surface of the silicon wafer which comes into contact with thesucking table.
 4. A method as set forth in claim 1, wherein the suckingtable is made of porous material and a sucking face of the sucking tablehas an outer diameter slightly larger than that of the body of thesilicon wafer after the bevel portion is removed.